Water-borne coatings continue to gain importance because of their low cost and also because of increasingly demanding regulations that limit the amount of solvent (volatile organic compounds, VOCs) that may be released to the atmosphere during coating application. Solvent-based systems are still used extensively, however, because they offer excellent performance.
Generally, there are two kinds of coatings: thermoplastic and thermoset. Polymer resins used in thermoplastic water-borne coatings, such as acrylic and SBR latexes, are usually prepared by emulsion polymerization. Thermoplastic coatings often lack adequate resistance to heat, water, detergents, solvents, or weathering because they are not crosslinked.
Thermoset coatings overcome some of these problems. Thermoset coatings derive from low molecular weight polymer resins such as polyether polyols, polyester polyols, hydroxy- or carboxy-functional acrylic resins (e.g., styrene/acrylic acid copolymers), and the like. The resins are dissolved or dispersed in water, and are mixed with a crosslinking agent and other optional components such as pigments, fillers, or flow modifiers. The coating is applied as a liquid, water evaporates, and crosslinking occurs at ambient or elevated temperature to give a cured film.
Even thermoset coatings, particularly those that use ester-containing polymer resins, can suffer from poor hydrolytic stability. Thus, ester-containing resins such as those derived from polyester polyols and polymer resins that contain a high proportion of pendant ester groups have limited usefulness in water-borne coatings. Recently, we described copolymer resins of acrylates and allylic alcohols or propoxylated allyl alcohols (see U.S. Pat. Nos. 5,480,943 and 5,475,073). These resins are useful for making thermoset coatings, but are better suited to high-solids, solvent-based coating systems because of the difficulty in making these resins water reducible.
We also described copolymers of propoxylated allyl alcohols and vinyl aromatic monomers as useful coating resins (see, e.g., U.S. Pat. Nos. 5,451,631 and 5,382,642). Because they normally contain a large proportion (50-90 wt. %) of recurring units of a vinyl aromatic monomer (typically styrene), these resins give coatings with high hydrophobicity and good resistance to water, detergents, or other chemicals. Although these resins can be formulated into many applications, including polyurethanes, alkyd resins, and melamine coatings, their use is limited to high-solids or solvent-based applications because they are not water-soluble or water-dispersible.
Other hydrophobic coating resins, such as styrene/acrylic acid copolymers, also give coatings with good water and detergent resistance. Total or partial neutralization of the acid groups of these resins gives a water-soluble resin useful in water-borne coatings. A disadvantage of these resins is that their curing rate is sometimes too slow because the acid groups are not all that reactive with typical crosslinking agents such as melamine compounds. If the cure is too slow, the coating stays tacky too long. A catalyst can be added to accelerate curing. However, if the coating cures too rapidly, solvent is trapped in the coating and causes bubbling or blistering. In addition, the lack of hydroxyl functional groups in styrene/acrylic acid copolymers makes them unsuitable for use in polyurethane coatings. Ideally, a resin would exhibit the hydrophobicity advantages of the styrene/acrylic acid copolymer resins, but would also have faster, yet more controllable curing rates.
In sum, there is a need for coating resins that have high hydrophobicity because these resins offer good resistance to water, acids, and weathering. Especially needed are hydrophobic resins that are suitable for use in water-borne coatings and inks. In addition, there is a need for resins that allow faster, but more controlled curing. Preferred resins could be used with a many types of crosslinkers or mixtures of crosslinkers to provide more flexibility in formulating the coatings and inks.